SoCal Offshore Weather Video

Thursday, October 25, 2012

The season of the Santa Anas

Having forecast weather in all parts of the country, I’ve found many areas have a unique weather feature or two worthy of its own name. In Colorado there are Chinook winds. In Seattle everyone has heard about the Puget Sound Convergence Zone. And of course here in Southern California we have our famous Santa Ana winds.

This is the time of year we start to see the weather pattern shift, a shift that produces those dry, gusty winds that blow the marine layer far out to sea, fan the flames of large wildfires, and “can” produce some fine fishing weather off the coast.

The typical Santa Ana winds pattern is shown below:

chrisdunn_santaana

Usually in the wake of large Pacific storms, cold air builds into the Great Basin (Nevada, Utah) and the circulation around the resulting center of high pressure brings wind out of the east/northeast into Southern California. The strength of the wind depends on the strength of the high pressure center, since all wind is the result of the atmosphere just trying to equalize things. The more out of balance it is, the more dramatic the result.

A weak to moderate offshore flow can often flatten out offshore seas by negating the prevailing winds, producing great weather past the Channel Islands. However, a strong Santa Ana can generate its own waves and there have been times when heavy surf has rolled into Avalon Harbor on the normally protected side of Catalina.

That “gradient wind” is strong enough, but when you add in the effects of our terrain, the wind actually accelerates! How is that possible?

Think of it this way: Water running through a garden hose flows at a constant rate with some pressure behind it, just like the wind blowing around the high pressure center over the Great Basin. But what happens when you constrict the opening by putting your thumb over it? The pressure builds and that water shoots out the smaller opening you have created!

The same principle (technically, it’s Bernoulli's Principle) explains why the wind increases in speed as it is squeezed through canyons and mountain passes. The same volume of air is moving but in a smaller space, so it has to speed up!
chrisdunn_santaana2

Check out this example of the northern Santa Monica Bay. When the wind is out of the north, it will tend to be locally stronger just below Zuma Canyon and Malibu Canyon. That is why with a Santa Ana wind forecast you’ll often hear or read something like “stronger below the canyons and passes.”

CBS 5 - KPHO

Posted By Sally On 10/25/2012 At 9:41 AM Comments (0) Trackbacks (0)

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